FIG. 1 is a schematic block diagram showing the configuration of a hydraulic system to which a method of controlling a flow rate of a variable displacement hydraulic pump for a construction machine in accordance with an embodiment of the present invention is applied.
A conventional hydraulic system applied to a hydraulic construction machine such as an excavator includes:
a manipulation lever (RCV) 1 that outputs a manipulation signal that is in proportion to a manipulation amount of the manipulation lever by a user;
a variable displacement hydraulic pump (hereinafter, referred to as “hydraulic pump”) 3 and a pilot pump 4 that are connected to an engine 2;
a hydraulic actuator (not shown) connected to the hydraulic pump 3;
a control valve 5 (for example, a spool for MCV is shown) that is installed in a discharge flow path of the hydraulic pump 3, and controls a start, a stop, and a direction change of the hydraulic actuator when it is shifted in response to the manipulation signal outputted from the manipulation lever 1;
a pilot pressure detection sensor 6 that detects a pilot signal pressure according to the manipulation of the manipulation lever 1;
a discharge pressure detection sensor 7 that detects a pressure of a hydraulic fluid discharged from the hydraulic pump 3; and
a controller 8 that controls a discharge flow rate of the hydraulic pump 3 in response to detection signals outputted from the pilot pressure detection sensor 6 and the discharge pressure detection sensor 7.
In the drawings, a non-explained reference numeral 9 denotes an electro proportional pressure reducing valve that generates a secondary signal pressure in proportion to a control signal applied thereto from the controller 8 to control a swivel angle of a swash plate of the hydraulic pump 3
FIG. 2 is a flow chart showing a method of controlling a flow rate of a hydraulic pump in accordance with the prior art.
In a first step S100, when a user manipulates the manipulation lever 1, a manipulation signal corresponding to a manipulation amount of the manipulation lever 1 is detected by the pilot pressure detection sensor 6, which in turn generates a manipulation amount detection_signal for application to the controller 8. Thus, the discharge flow rate Q1 required by the hydraulic pump 3 in proportion to the manipulation amount of the manipulation lever 1 is calculated by using a relationship between the manipulation amount of the manipulation lever 1 and the volume of the hydraulic pump 3.
In a second step S200, a discharge pressure of the hydraulic pump 3 is detected by the discharge pressure detection sensor 7, which in turn generates a discharge pressure detection signal corresponding to the discharge pressure for application to the controller 8. Thus, a maximum dischargeable flow rate Qmax within a range that does not exceed a specific horsepower or torque of the hydraulic pump 3, relative to the detected discharge pressure is calculated by a calculation equation.
In a third step S300, the discharge flow rate Q1 required by the hydraulic pump 3 in proportion to the manipulation amount of the manipulation lever 1 is compared with the maximum dischargeable flow rate Qmax within the range that does not exceed the preset value.
If it is determined in the third step S300 that the discharge flow rate Q1 required by the hydraulic pump 3 is less than the calculated maximum dischargeable flow rate Qmax, the program proceeds to a fourth step S400 where the discharge flow rate of the hydraulic pump 3 is controlled in proportion to the manipulation amount of the manipulation lever 1.
On the contrary, if it is determined in the third step S300 that the discharge flow rate Q1 required by the hydraulic pump 3 exceeds the calculated maximum dischargeable flow rate Qmax, the program proceeds to a fifth step S500 where the discharge flow rate of the hydraulic pump 3 is controlled to be the maximum dischargeable flow rate Qmax within the range that does not exceed the preset value.
The method of controlling the discharge flow rate of the hydraulic pump 3 as described above has the following advantages.
First, the discharge flow rate of the hydraulic pump 3 is increased in proportion to the manipulation amount of the manipulation lever 1 by the user, and the discharge flow rate of the hydraulic pump 3 is minimized in case of no manipulation of the manipulation lever 1, thereby reducing a loss or waste of hydraulic energy.
Second, in the case where the discharge pressure of the hydraulic pump 3 exceeds a preset value determined within a range that does not exceed a torque or horsepower allocated to the hydraulic pump 3, a flow rate as much as a pressure level that exceeds the preset value is limited (shown in FIG. 6), thereby reducing the flow rate determined in the first step.
In the case where the discharge pressure of the hydraulic pump 3 is controlled by the above-mentioned method, i.e., the discharge pressure of the hydraulic pump 3 is controlled by a mechanical mechanism or an electronic control device to limit the torque or horsepower, if the discharge pressure of the hydraulic pump 3 is high, there occurs a problem in that the control range of the manipulation lever 1 by the user is shortened. Particularly, even in the case where a more precise work is required such as the lifting work of heavy materials, the control range of the manipulation lever 1 is shortened, which makes it difficult to ensure a more precise manipulability.
FIG. 3 is a graph showing a correlation between the discharge pressure and the volume or flow rate of the hydraulic pump when the torque or horsepower of the hydraulic pump is limited. FIGS. 4 and 5 are graphs showing the control method of the flow rate of a hydraulic pump in accordance with the prior art, i.e., graphs showing a correlation between the manipulation amount of the manipulation lever and the discharge volume or flow rate of the hydraulic pump in points where the discharge pressures of the hydraulic pump are P1 and P2.
As shown in FIG. 4, the discharge flow rate of the hydraulic pump is increased in proportion to the manipulation amount of the manipulation lever within a range of the allowable discharge flow rate at a point where the discharge pressures of the hydraulic pump is P1.
In the meantime, as shown in FIG. 5, the discharge flow rate of the hydraulic pump is not increase any more in a range beyond a control range (b) even in the case where the manipulation amount of the manipulation lever is increased, at a point where the discharge pressures of the hydraulic pump is P2. Thus, there occurs a problem in that the control range (b) of the manipulation lever is relatively short as compared to a control range (a) of the manipulation lever as shown in FIG. 4, leading to a deterioration of manipulability.
As shown in FIG. 6, in the case where the manipulation amount of the manipulation lever is 50% or 75% of the maximum manipulation amount, if the discharge flow rate of the hydraulic pump exceeds the preset value determined to limit the torque or horsepower of the hydraulic pump, a flow rate corresponding to the excess portion is limited by a control diagram. As such, the control range in the case where the manipulation amount of the manipulation lever is 75% of maximum manipulation amount is shorter than that in the case where the manipulation amount of the manipulation lever is 50% of maximum manipulation amount, which makes it impossible to precisely manipulate the manipulation lever during the lifting work of heavy materials.